Electrical equipment uses ever higher radio frequencies and wider signal bandwidth for wireless communications, imaging, data transmission and other applications. Often this equipment requires a computer to process the signals to extract information or process the signals with other algorithms. This information extraction or processing requires the conversion of the analog radio frequency signals into digital form. Conversion of analog signals into digital generally requires a sample and hold on the analog signal and quantization of the analog signal into a digital one. These conventional analog to digital converters are limited by the bandwidth of the device technology employed and by timing issues. As a result, the conversion of high frequency analog signals in the tens of gigahertz is difficult with present technology. In addition, the noise imparted to the digital signal is proportional to the dynamic range of the quantization device. The greater the resolution of the quantizer, the less noise is added to the system by the conversion from analog to digital signals. Moreover, some applications benefit from encoding the analog signal into Gray code instead of straight binary as a way to add some noise immunity. Still other applications benefit from the analog signal being converted into a two's complement binary number.
The invention described herein allows the quantization of analog signals in the tens of gigahertz range to digital signals using optical encoding where the number of bits is limited primarily by the ability to construct optical reflectors on a plurality of optical waveguides.
G. C. Valley in “Photonic analog-to-digital converters” Optics Express, v15, no. 5 pp 1955-1982, 2007 surveyed the field and concluded that the maximum number of bits that could be obtained is four, based on “An optical analog to digital converter—Design and Analysis” by H. Taylor, IEEE Journal of Quantum Electronics v 15 pp 210-216 incorporated by reference herein in its entirety. Valley concluded Taylor's scheme required too high a voltage for achieving a π-phase shift by the optical modulator. This is commonly known as the Vπ problem in that the voltage needed to shift the phase multiple times for the Least Significant Bit modulator by π, first is proportional to the input light wavelength and inversely proportional to the cube of the refractive index multiplied by the effective electrooptic coefficient, and second can be large enough to cause the breakdown of the dielectric material separating the electrodes, as well as being difficult to generate because of the high frequency. Moreover, Taylor's scheme is limited to periodic variations in transmission bands because his design relies on interferometric modulators.
The design herein avoids the Vπ problem of Mach-Zehnder or Fabry-Perot interferometers. By using gratings or other suitable reflectors designed to reflect particular wavelength bands with a reasonable voltage, the need for Mach-Zehnder or Fabry-Perot interferometers is eliminated. The use of gratings also means the design is not limited to periodic variations in transmission bands.
The basics of obtaining spectral shifts with gratings on optical waveguides is described in U.S. Pat. No. 6,640,020 (S. Ionov, Method and Apparatus for Electro-Optic Delay Generation and Optical Signals) incorporated herein by reference in its entirety and “Fabrication and Application of Holographic Bragg Gratings in Lithium Niobate Channel Waveguides” J. Hukriede, D. Runde, D. Kip, Journal Physics D: Applied Physics v36 pp R1-R16, 2003, incorporated herein by reference in its entirety.
While using intensity modulation in photonic analog to digital quantizers is know in the art, using a plurality of cascaded reflection gratings on one waveguide with an encoded spectra that are shifted by application of an electric field to reflect, or not reflect, a laser beam and then detect the reflected light as either a binary 1 or 0, has not been done. The gratings or reflectors herein are used as intensity modulators.
There is a need to quantize high frequency analog signal into high resolution straight binary, Gray coded, or two's complement digital signals to enable further processing of high frequency analog signals.